The turbochargers used for the internal combustion engines of automobiles and ships include a compressor impeller that compresses and supplies air into the internal combustion engine by rotating at high speed. The compressor impeller can reach temperatures as high as about 150° C. during its high-speed rotation, and receives high stress, such as the torsional stress from the rotating shaft, and the centrifugal force, near the center of rotation, particularly at the disc portion.
Various materials are used for the compressor impeller according to the required performance of the turbocharger. Hot forged materials of an aluminum alloy machined into an impeller shape are typically used in large-scale applications such as ships. Mass production efficiency and costs are more important in relatively smaller applications such as in automobiles (e.g., cars, and trucks), and boats. Such applications commonly use easily castable aluminum alloys of primarily silicon additive such as JIS-AC4CH (Al—7% Si—0.3% Mg alloy), ASTM-354.0 (Al—9% Si—1.8% Cu—0.5% Mg alloy), and ASTM-C355.0 (Al—5% Si—1.3% Cu—0.5% Mg alloy) of desirable castability. These materials are then cast with a plaster mold by using techniques such as low-pressure casting, vacuum casting, and gravity casting, and are strengthened by a solution treatment or an aging treatment before use. A basic method of such procedures is disclosed in detail in Patent Document 1.
Lately, the need for high-speed turbochargers has increased with the increase in the demand for higher compression ratios of air necessitated by smaller engines, higher output, and increased exhaust recirculation. However, faster rotation speeds increase the amount of heat generated by air compression, and at the same time increase the temperature of the exhaust turbine impeller. This heat is conducted to increase the temperature of the compressor impeller. It has been found that conventional compressor impellers made of easily castable aluminum alloys of primarily silicon additive tend to cause problems such as deformation and fatigue failure during use, and fail to keep rotating normally. Specifically, these existing compressor impellers have an operating temperature of at most about 150° C., and there is a strong need for the development of a compressor impeller that can withstand an operating temperature of about 200° C. to meet the demand for high speed rotations.
It may be possible to use an aluminum alloy composition of more desirable high-temperature strength, for example, such as JIS-AC1B (Al—5% Cu—0.3% Mg alloy). However, as described in Patent Document 2, the problem of such an alloy is that the molten metal lacks desirable fluidity, and tends to cause misruns (underfilling) of the molten metal in thin portion of blade parts when used to make articles that have complex shapes and thin blade parts such as in compressor impellers.
Patent Document 2 addresses this problem by proposing a method that uses an Al—Si easily castable alloy such as AC4CH for the blade part for which misruns of a molten metal are of concern, and an Al—Cu high-strength alloy such as AC1B for the boss and disc parts that are connected to the rotating shaft and thus require strength. These are coalesced by being poured in two separate portions to form a compressor impeller.
Patent Document 3 proposes a method that uses an alloy of desirable castability for the blade part, and in which a strengthened composite material prepared by impregnating a strengthening material such as a 25%-B (boron) aluminum whisker with aluminum is used for the stressed boss portion and the central portion of the disc part. These are then joined to each other to form a compressor impeller.
Patent Document 4 proposes a method in which a blade part and a boss part (and a disc part) are joined to each other by friction welding. However, methods such as this that use different materials for different parts are problematic in terms of productivity and cost, and are currently not usable in industrial applications.
Patent Document 5 addresses the problem of using different materials by proposing a compressor impeller that can be cast from a single alloy, specifically an Al—Cu—Mg-base alloy for which the additive elements and the combination range of these elements are optimized. The resulting compressor impeller has a proof stress value of 250 MPa or more at 180° C. Patent Document 6 proposes improving the casting yield by controlling the crystal grain size of an Al—Cu—Mg-base alloy through optimization of the additive elements and the combination range of these elements. The compressor impeller has a proof stress value of 260 MPa or more at 200° C.
However, a problem remains that the products of the single alloy casting using the Al—Cu—Mg-base alloy still, need to stably withstand high temperatures in the vicinity of 200° C. over extended time periods if these were to be used for ever faster turbochargers. Another unsolved problem is that the casting yield needs to be improved for stable production.